1. Field of the Invention
This invention relates to the field of current sense amplifiers, particularly those designed to operate with a low supply voltage.
2. Description of the Related Art
Current sense amplifiers produce an output that varies with a particular current of interest. Such amplifiers are typically connected across a "sense resistor" through which the current of interest, i.e., the "sensed current", flows. The amplifiers produce a current or voltage output that is proportional to the sensed current.
A known current sense amplifier is shown in FIG. 1. A sense resistor R.sub.s is connected to carry the current of interest i.sub.s, creating a voltage at a node 10 that increases with the sensed current. A diode-connected transistor 12, shown here as an npn bipolar transistor though pnp's and field-effect transistors (FETs) are used as well, is biased with a current source i1 and is connected to node 10 via a resistor R.sub.a. A second transistor 16 shares a common base connection with transistor 12, and its emitter is connected to ground through a resistor R.sub.b. The current sense amplifier output i.sub.out is taken at the collector of transistor 16.
An increasing i.sub.s increases the voltage at node 10, as well as at the emitter of transistor 12. This raises the voltage at the bases of transistors 12 and 16, boosting the drive to transistor 16 and thereby increasing i.sub.out. For this circuit, output i.sub.out is related to the sensed current i.sub.s as follows: EQU i.sub.out =i.sub.s (R.sub.s /R.sub.a)+i1(R.sub.b /R.sub.a)-(V.sub.T ln(i.sub.out /i1))/R.sub.a
Unfortunately, the output current produced by this current sense amplifier it is not very accurate. The output current equation's "i1 (R.sub.b /R.sub.a)" and "(V.sub.T ln(i.sub.out /i1))/R.sub.a " terms are error terms, which show that the i.sub.out is greatly affected by changes in current source i1 and by changes in temperature.
Another current sense amplifier is disclosed in U.S. Pat. No. 5,627,494 to Somerville. This amplifier is referred to as a "high side" amplifier, because it is arranged to sense a current that is near the supply voltage, as opposed to a "low side" amplifier which senses a current near ground. Though more accurate than the amplifier of FIG. 1, this circuit (as shown in FIG. 3 of the patent) requires seven transistors, two diodes, five resistors and a current source. The circuit has two transistors (43, 44) in its output stage and another (35 or 36) in its input stage, and as such requires a supply voltage equal to at least V.sub.out +2(V.sub.be)+V.sub.sat +V.sub.R1 +V.sub.SENSE to operate. Using typical values, a supply voltage of at least 2 volts is needed to operate this circuit, or more if V.sub.OUT is larger.
Present trends in analog circuit design are toward ever lower supply voltages. A need exists for a current sense amplifier that is capable of operating with a low supply voltage while producing an accurate output current.